21217555 has 4 divisors (see below), whose sum is σ = 25461072. Its totient is φ = 16974040.

The previous prime is 21217549. The next prime is 21217561. The reversal of 21217555 is 55571212.

It is a semiprime because it is the product of two primes.

It is an interprime number because it is at equal distance from previous prime (21217549) and next prime (21217561).

It is a de Polignac number, because none of the positive numbers 2^k-21217555 is a prime.

It is a Duffinian number.

It is an unprimeable number.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2121751 + ... + 2121760.

It is an arithmetic number, because the mean of its divisors is an integer number (6365268).

Almost surely, 2^21217555 is an apocalyptic number.

21217555 is a deficient number, since it is larger than the sum of its proper divisors (4243517).

21217555 is an equidigital number, since it uses as much as digits as its factorization.

21217555 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4243516.

The product of its digits is 3500, while the sum is 28.

The square root of 21217555 is about 4606.2517299861. The cubic root of 21217555 is about 276.8418730229.

Adding to 21217555 its reverse (55571212), we get a palindrome (76788767).

The spelling of 21217555 in words is "twenty-one million, two hundred seventeen thousand, five hundred fifty-five".